1. Field of the Invention
The invention relates to a microlens structure and more particularly to a microlens structure having a multi-layered refraction film on a microlens element and a method for forming the multi-layered refraction film.
2. Description of the Related Art
In image sensor devices, a microlens structure is usually disposed on an image sensor chip. The conventional microlens structure has a microlens element and only one layer of an Silicon oxide film with one refraction index is formed on the microlens element.
In a conventional method, the silicon oxide film is formed by coating a liquid oxide material onto the microlens element and then performing a hard baking process to the liquid oxide material to form the silicon oxide film on the microlens element. However, the silicon oxide film formed by this conventional method has a poor step coverage ratio of less than 60%, thus the silicon oxide film is easy delaminated from the microlens element.
In another conventional method, the silicon oxide film is formed by a plasma enhanced chemical vapor deposition (PECVD) process using a process temperature higher than 200° C., a radio frequency (RF) power higher than 300 watt, a rich O2 environment of O2 flow rate of 2000 standard cubic centimeter per minute (sccm) and a gas source of silane (SiH4) and N2O or a gas source of tetraethyl orthosilicate (TEOS) and O2. However, the high process temperature, the high RF power and the rich O2 environment used in the conventional PECVD process damages the organic material layer under the microlens element. Also, the silicon oxide film formed by the PECVD process using the gas source of SiH4 and N2O has a poor step coverage ratio of less than 70%. The silicon oxide film formed by the PECVD process using the gas source of TEOS and O2 has a poor step coverage ratio of less than 80%. Thus, the silicon oxide film is also easily delaminated from the microlens element.
Furthermore, the sensitivity of the image sensor device cannot be greatly enhanced by the silicon oxide film with one refraction index. The stress of the silicon oxide film with one-layered structure cannot be adjusted to get optimum stress performance for a large process window of the microlens structure.
Therefore, a microlens structure and a fabrication method of the microlens structure which overcomes the above problems are desired.